be (not) supposed to (do something)

To be expected to do something. Often used in the negative to indicate that one is expected to avoid doing something. I'm supposed to watch my little sister tonight, so I can't go out with you guys. You're not supposed to play with Dad's tools when he's not home! This new software is supposed to make things a lot more efficient, but I find it so confusing that everything is taking me twice as long!

(not) be supˈposed to (do something)

1 (not) be expected or required to do something by rules, the law, an agreement, etc: She’s supposed to do an hour’s homework every evening. ♢ We’re not supposed to be at the party for an hour yet.
2 (used only in negative sentences) be not allowed to do something: You’re not supposed to walk on the grass.